The present invention relates to a process for the production of high molecular weight oxymethylene copolymer having a predetermined comonomer content. The process provides the desired product by compensating for the presence of impurity which contains at least one active proton.
Oxymethylene polymers having successively recurring oxymethylene units are useful in the production of molded and extruded objects. Such polymers have excellent physical properties and produce strong molded objects. The thermal stability of oxymethylene polymers may be enhanced by the incorporation into the polymer molecule of interspersed comonomeric units having adjacent carbon atoms. It is desirable to incorporate such comonomeric units in controlled amounts since excessive proportions of interspersed comonomeric units impair the physical properties of the polymers while insufficient amounts of the comonomeric units fail to provide the required thermal stability.
The amount of comonomer to be incorporated into an oxymethylene polymer for optimum enhancement of thermal stability without impairment of physical properties will vary depending upon the particular comonomer used and depending upon the properties desired in the copolymer. For copolymers having oxymethylene groups interspersed with groups derived from a cyclic ether, such as the copolymers described in U.S. Pat. No. 3,027,352, the proportion of comonomer units generally varies from about 0.1 to about 15 percent of the total monomeric units. The preferred copolymers contain from about 0.1 to about 5 percent of comonomer units.
In said U.S. Pat. No. 3,027,352, which is herein incorporated by reference, stable copolymers having successively recurring oxymethylene groups are prepared by copolymerizing trioxane with a cyclic ether. In accordance with the present invention stable copolymers are prepared by copolymerizing formaldehyde with a cyclic ether or with other comonomers having adjacent carbon atoms, as explained more fully hereinbelow.
The copolymerization of formaldehyde with comonomers having adjacent carbon atoms presents problems which the copolymerization of trioxane does not present. It has been found that while it is relatively easy to produce copolymers of formaldehyde having relatively high proportions of comonomeric units incorporated therein, it is exceedingly difficult to prepare copolymers having the low desired proportions of comonomeric units which provide stability but, at the said time, do not impair the physical properties of the homopolymer.
The prior art has attempted to copolymerize formaldehyde with other monomers in a liquid medium wherein formaldehyde and the desired comonomers are continuously introduced in admixture. In such polymerization systems it has been found that when the admixture contains a relatively high proportion of comonomer the copolymerization proceeds satisfactorily but produces a copolymer containing an excessive amount of comonomeric units for optimum physical properties. On the other hand, when the admixture contains a relatively low proportion of comonomer the prior art polymerization processes produce formaldehyde homopolymer while the comonomer either remains unreacted or reacts only with itself to produce a second homopolymer in admixture with the formaldehyde homopolymer.
The prior art has also attempted to copolymerize formaldehyde with another monomer by feeding formaldehyde into a reaction zone which already contains, dissolved in an inert solvent, all of the comonomer to be reacted. In such polymerization systems, it has been possible to control the total amount of comonomer in the copolymer product, but the distribution of comonomer has been unsatisfactory. It appears that the initial polymerization takes place in a reaction zone containing excessive concentrations of comonomer so that the comonomer combines with itself to produce the comonomer chain segments within the molecule thereby providing poor distribution of comonomer units in the final polymer.
U.S. Pat. No. 3,376,361 discloses a method of producing a polymer having successively recurring oxymethylene groups interspersed with divalent comonomeric groups having adjacent carbon atoms. The process comprises passing a mixture of formaldehyde and a comonomer into a reaction zone in the presence of an inert liquid solvent and a cationic catalyst. The comonomer has adjacent carbon atoms and is capable of copolymerization with formaldehyde in the presence of said cationic catalyst. The mixture is reacted to produce a prepolymer and the prepolymer is thereafter reacted with additional formaldehyde.
In order to consistently obtain oxymethylene copolymers having a high molecular weight and having a desired, predetermined comonomer content, such prior art formaldehyde copolymerization processes have had to employ pure reactants, solvent, and catalyst so that the overall reaction system contained less than approximately 10 p.p.m. impurity (as hereinafter defined).
However, commercial solvents and formaldehyde streams commonly contain significant quantities of impurity, such as water, formic acid, methanol, etc. When such commercial solvents and reactants have been utilized in formaldehyde copolymerization processes, the molecular weight of the oxymethylene copolymer product has tended to be depressed, and the comonomer content of the copolymer has been difficult to control.
It is therefore an object of the present invention to provide a process for the production in the presence of impurity of oxymethylene copolymer having high molecular weight and a predetermined comonomer content.
It is also an object of the present invention to provide a process for the production of oxymethylene copolymer having high molecular weight and a predetermined comonomer content which compensates for the presence of impurity.
It is also an object of the present invention to provide a process for the production of oxymethylene copolymer having high molecular weight, a predetermined comonomer content, and randomly distributed comonomer units.
It is also an object of the present invention to provide a process for the production in the presence of impurity of oxymethylene copolymer having high molecular weight and a predetermined comonomer content, the process being more economical than prior art formaldehyde copolymerization processes.